In recent years, sewage from homes, waste water from manufacturing plants, chemical residues from farms, and feed residues from fish farms have polluted water in rivers, lakes, sea and other bodies of water, and such pollution of water can be measured by the increase in BOD (biochemical oxygen demand), COD (chemical oxygen demand) and other values. When water is polluted beyond a certain extent, underwater plants and animals are destroyed, and when such polluted water is used as a water resource for human consumption, living environments of humans are also destroyed. Therefore, it is now necessary to purify waste water from various sources on the one hand and to purify large bodies of water into which polluted water has mixed in.
As a method for purifying large bodies of contaminated water and waste water from various sources, it is known to form a processing layer by using gravel, a honeycomb structure or anthracite, and allow contaminants heavier than water to be caught in the processing layer. However, according to this method of purifying water, because a fixed filtering or other processing layer is not effective in carrying air and bacteria therein, it has not been possible to achieve a sufficient water purifying effect. In short, this process is not efficient enough to purify a large body of water in an economically feasible fashion. Additionally, to prevent the clogging of such a processing layer due to deposition of foreign matters, it is necessary to backwash the processing layer at regular intervals, and it is therefore not possible to carry out the process of purifying water as a truly continuous process.
Furthermore, because the processing layer is fixed, it is necessary to supply an excessive amount of air to the processing layer for bacteria effective in purifying water to grow, and the resulting increase in power consumption increases the operating cost of the water purifying process. This method is also not efficient enough to be effective in purifying large bodies of water because its capability to reduce BOD and COD is again quite limited.
The inventor has discovered that rivers and lakes which are known to have a natural capability to purify themselves can purify the water in relatively short time only when the extent of contamination is within certain limit, and will lose such a capability when the extent of contamination exceeds a certain limit. For instance, in severely contaminated rivers and lakes, the bottom is covered by mud which is in an anaerobic condition, and this severely restricts the natural or biological water purifying process. Therefore, in order to purifying large body of water, any attempt to simply purify the water either physically or biologically is inadequate in most cases, and it is necessary to take a measure which will promote the natural biological water purifying process. In other words, it is necessary to ultimately rely on the natural biological process to purify a large body of water.